Reactive transformers



L. R. HERMAN ETAL 3,128,443

I April 7, 1 964 REACTIVE TRANSFORMERS 2 Sheets-Sheet 1 Filed'Aug. 6, 1958 v n e a n ma m Wimp HJ A m /.w r m h mm T m LH h.

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p 7, 1954 L. R. HERMAN ETAL 3,128,443

REACTIVE TRANSFORMERS Filed Aug. 6, 1958 2 Sheets-Sheet 2 F "'I 36 I 3! I G I rvfim I I l 32 33 I 35 3O 0 i I I I I Fig. 3

INVENTORS LuIher R. Herman Howard T. Jones Their AHorney United States Patent 3,123,443 REACTIVE TRANFORMERS Luther R. Herman, Flat Rock, and Howard T. Jones, Hendersonviile, Nail, assignors to General Electric Company, a corporation of New York Filed Aug. 6, I958, Ser. No. 753,460 3 Claims. (Cl. 336-460) This invention relates to reactive transformers and, more particularly, to a transformer and transformer core construction useful, for example, in regulator ballasts for are discharge devices. Although the transformer constructions disclosed herein are primarily intended for use in ballasting the operation of arc lamps and will be described principally from the standpoint of such utility, it should be understood that these teachings will also have applicability to such fields as arc welding and voltage regulation when it is desired to make use of a transformer having considerable leakage reactance.

The transformer portion of a regulator ballast can be made in many conceivable ways. The basic requirements are, however, that independent primary and secondary electrical circuits should be mutually linked by a magnetic circuit and that another magnetic circuit should be provided to link either the primary or the secondary circuit without necessarily linking the other. In addition, the secondary electrical circuit should be linked by a saturable magnetic circuit so that the flux linkages of the secondary will not be a linear function of the primary potential.

Within these basic requirements, it is an object of this invention to provide a reactive transformer whose core is formed in such a manner as to make maximum effective utilization of core winding techniques to reduce the total amount of magnetic material required while maintaining superior electrical characteristics.

By way of a brief summary of but one embodiment of the present teachings, the transformer core is formed in the shape of a closed magnetic circuit loop by winding a ribbon of magnetic material layer upon layer until a sufiicient thickness is built up. The core is then out by sawing or otherwise into unequal parts having mating butt joints. The larger of the two portions of the core loop is provided with a saturable area of reduced cross section by sawing a slot partially through the core normal to the edge of the laminations. Precision wound primary and secondary electrical windings are then assembled inductively on the core loop in such a way that the secondary windings are on that end of the core loop on which the restriction occurs and such that the butt joints lie directly beneath the primary windings.

To complete the reactive transformer, a loose magnetic shunt is then created between the primary and secondary ends of the core loop by placing a stack of laminations on each side of the core loop spanning the central opening so that the laminations of the loop are edge to edge with those of the shunt. To prevent magnetic saturation of adjacent laminations, layers of paper or other composition are inserted between the magnetic shunts and the core loop, and the shunts are then fastened in place simply by tying them together, in effect clamping them to the core loop. A completed reactive transformer formed in this manner makes very efficient use of magnetic materials, since all the stock is initially in the form of strip and no irregular punchings productive of waste are employed. Furthermore, the unique placement of the component parts results in electrical characteristics at least as good as those of previous units. The result, therefore, is a comparatively inexpensive product with no sacrifice in performance.

Although the scope of this invention should not be lim- 3,128,443 Patented Apr. 7, 1964 ICC ited except by a fair interpretation of the appended claims, further details of the invention as Well as additional objects and advantages may be more readily understood in connection with the accompanying drawings, wherein:

FIGURE 1 is an isometric view of a loosely shunted transformer constructed in accordance with these teachmgs;

FIGURE 2 is a plan view, with the electrical windings cut away, of the transformer illustrated in FIGURE 1;

FIGURE 3 is an equivalent circuit of a regulator ballast for are discharge lamps illustrating the electrical characteristics of a reactive transformer constructed in accordance with these teachings;

FIGURE 4 is a plan view similar to FIGURE 2 but with the core structure partially cut away illustrating an alternative embodiment of the present invention; and

FIGURE 5 is an isometric view of still another embodiment of the present teachings.

In FIGURES 1 and 2 the core structure of the reactive transformer is shown as comprising a generally rectangular core loop 1 of magnetic strip material Wound layer upon layer until a sufficient cross sectional area is built up. After Winding of the core loop, the superposed laminations may be bonded together by any known technique to prevent their subsequent separation and the loop is then cut into two unequal sections 2 and 3. In that portion 2 of the core loop which is to carry the secondary windings, a restriction in the cross sectional area of the core is introduced to provide a saturable area, in this instance, by cutting a narrow slot 4 in the edge of the core to a depth of approximately one-third the width of the laminations. Cutting of the core into two sections permits the placement thereon of prewound precision electrical windings formed, in this instance, of a pair of primary windings 5 and 6 and a pair of secondary windings '7 and 8, each insulated from the transformer core by a layer of insulating material such as the paper insulators 9 shown. It is to be noted that the butt joint 10 formed by juxtaposing the two parts of the core loop occurs in that part of the magnetic circuit bearing the primary windings and, in fact, lies immediately underneath the primary windings. The two portions of the core loop are fastened together by the use of a steel strap 12 of a type conventionally used for this purpose.

In the reactive transformer shown in FIGURES l and 2 a loose magnetic shunt is added to the basic core loop by the provision of a pair of stacks of laminations 13 and 14, spaced from the core loop such as by insulating layers 15 and 16. These laminations are placed beside the core loop in edge-to-edge relationship with the layers of the loop to span the central opening, thus providing a leakage path for the primary and secondary portions of the core loop. Although omitted from FIGURES 1 and 2 for clarity, means for holding the shunts in place on the core loop prior to impregnating with a potting compound can take the for-m1 of tape, for example, wound about the center or ends of the two stacks of 'l-aminations, effectively clamping them to the core loop. This disposition of the magnetic shunt overcomes a problem which until now has discoumaged the use of strip winding techniques in the formation of reactive transformer cores. It has been customary to place the shunt in the plane of the basic core loop. But if such a shunt were placed within the plane of a strip wound core, most of the flux shunted through it would necessarily have to pass through several adjacent layers of the laminations inthe core loop in a direction normal to the lam-inations before ever entering the shunt, an eventuality that would materially increase core losses and decrease the effectiveness of the shunt. For this reason, as much as for any other, the core structures of prior reactive transformers have generally been formed from stacked punchings which, although somewhat wasteful of magnetic material, have had satisfactory electrical characteristics. By placing a laminated shunt beside the wound core loop instead of in the same plane and orienting its laminations edge to edge with those of the core loop, the effectiveness of the shunt is preserved, and it becomes feasible to strip wind the core loop. The resulting core structure is, therefore, very economical because it can be formed entirely of strip materials with no appreciable waste.

A regulator ballast for an arc discharge device, such as a mercury vapor lamp or a fluorescent lamp, is intended to maintain a nearly constant current into the lamp load with variations of supply potential. When the lamp is operated at or near its rated power, although the potential across it remains nearly constant, variations in the lamp current are reflected in the energy output of the lamp. The lamp current and hence the energy output can be maintained nearly constant by applying a constant voltage to the lamp and by limiting the amount of current which the lamp is permitted to draw. As its basic approach to these design considerations, the reactive transformer which is the subject of this invention provides a nonlinear transformer coupling between .a source of power and a load circuit including, for example, an arc lamp and a series capacitor. The series capacitor, in addition to its current limiting function, provides the necessary leading current through the nonlinear reactive transformer so that a constant output voltage may be obtained. For a better appreciation of these and other factors, FIGURE 3 shows an equivalent circuit of the transformer which is the subject of this invention operatively connected to ballast the operation of an arc lamp 30. Those elements bounded by the dashed-line rectangle represent the effective impedance elements resulting from the transformer construction described and include an ideal transformer 31 having a turns ratio of a preselected value. The re actance 32 represents the primary magnetizing reactance referred to the secondary circuit. The reactances 33 and 34 represent, respectively, the secondary magnetizing rcactance andthe total leakage reactance. The ballast is completed by the connection in the output circuit of a shunt capacitor 35 across the output leads and a series capacitor 36 in series with the load.

With the transformer construction previously described, it is possible to vary each of the effective reactances to obtain optimum characteristics for a given installation. The loose shunts placed between the primary and secondary coils provide the effective leakage reactance 34 which must be of a value in proportion to the series capacitor reactance. The 'value of this leakage reactance may be adjusted without altering the characteristics of the other effective reactances simply by varying the cross sectional area of the shunt and/or the width of the air gap between the edges or the shout and the edges of the core loop. The elfective secondary magnetizing reactance 33 provides a nonlinear saturating effect that affects both the regulation of load current with variations in input potential and the crest factor of the load current. The characteristic of this reactance is controlled principally by the core restriction placed on the secondary side of the core. The dimensions of the restriction, which may be a hole or a slot, and the type of steel employed control the actual characteristic of the secondary magnetizing reactance. Normal variations in the primary voltage affect only the degree of saturation of the secondary magnetizing reactance, and thus the voltage output of the ballast remains nearly constant.

By placing the butt joint in the primary portion of the core loop it affects only the primary magnetizing reactance 32, and the effective gap may be adjusted to control the input power factor. If this joint were located entirely or partially in the secondary core portion, it would affect the secondary magnetizing inductance, thereby adversely affecting regulation and increasing transformer losses. The shunt capacitor, which may also be termed a peaking capacitor, is used to increase the peak open circuit voltage above the turns-ratio voltage and should preferably be of a value to resonate with the leakage reactance at slightly above the third harmonic frequency. Although shown as connected across the entire output of the ballast transformer, the pea-king capacitor may also be connected across only a part of the secondary turns to reduce to some extent the open circuit voltage supplied by the ballast.

Another reactive transformer embodying the principles of this invention is shown in FIGURE 4 as including a pair of strip wound, butt jointed cores 4d and 42, generally similar to the cores shown in FIGURES 1 and 2. The core restriction, which in this instance is provided by holes 43 and 44, drilled through one side of th longer part of each core loop, is placed in the secondary portion of the core immediately underneath secondary windings 45. On the upper portion of the core loops the primary win ings 46 cover two of the butt joints 47. In this case, the magnetic shunt is provided by two stacks of laminations, only one of which, 48, is shown in the figure, spanning both core loops between the primary and secondary windings. This construction makes use of two core loops but only one set of primary and secondary windings. It offers most of the features of the transformer design described in FIGURES l and 2 along with the added advantage of an improved heat transfer from the core outward because of a more exposed core area.

In some instances it may not be desired to make use of prewound electrical windings to be inserted on the strip wound core after cutting the core into two parts. An alternative embodiment of these teachings, shown in FIGURE 5, makes use of a generally toroidal transformer construction with the core 51 wound upon a circular mandrel. To form the area of reduced cross section, a hole or a slot may then be placed in that portion of the core which is to form a part of the secondary magnetic circuit. The core may then be insulated to prevent it from shorting out any of the electrical windings to be added. Thereafter, primary and secondary windings 52 and 53, respectively, are wound toroidally on opposite sides of the circular core, a space being left between them for insertion of the magnetic shunts. In the space which is left a pair of stacks of laminations 54 and 55 may then be placed in edge-to-edge relationship with the laminations of the strip wound core spanning the central opening in the core. These stacks of laminations may be held in place by suitable binding means such as tapes 56. This construction requires the generally more expensive toroidal windings but eliminates the necessity of a butt joint in the core loop.

Although certain preferred embodiments have been described herein, it is to be understood that these are illustrative in nature and not necessarily limiting upon the scope of these teachings in their broader aspects. Certain important operations, such as the annealing step necessary to develop the required characteristics of the magnetic materials employed, have not been mentioned herein, since they form no part of the present invention and because they are familiar techniques to those skilled in the art to which this invention pertains. Many additional variations within the scope of the appended claims will doubtless occur to persons skilled in the transformer and ballast arts.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A reactive transformer comprising a magnetic circuit loop formed of a plurality of laminations of magnetic material bent fiatwise to form a closed path about a central window, a fiat stack of lamiuations placed against the side of said circuit loop defined by the edges of the laminations of said circuit loop across said central win dow in edge-to-edge relationship with the laminations of said loop, said loop being provided with a butt joint on one side of said stack of laminations, a primary electrical winding on said one side of said stack of laminations and overlying said butt joint, and a secondary electrical winding on said loop on said other side of said stack of laminations.

2. A reactive transformer comprising a magnetic circuit loop formed of a plurality of laminations of magnetic material bent flatwise to form a closed path about a central window, and a fiat stack of laminations placed against the side of said circuit loop defined by the edges of the laminations of said circuit loop across said central window in edge-to-edge relationship with the laminations of said loop, said loop being provided with a butt joint on one side of said stack of laminations and with an area of reduced cross-section on the other side of said stack of laminations, a primary electrical winding on said loop on said one side of said stack of laminations and overlying said butt joint, and a secondary electrical winding on said loop on said other side of said stack of laminations.

3. A transformer comprising a magnetic core and spaced primary and secondary electrical windings inductively associated therewith whereby said magnetic core is divided into primary and secondary portions, said magnetic core comprising two unequal length sections of superimposed laminations of strip material secured together to form a butt joint in the primary portion so References Cited in the file of this patent UNITED STATES PATENTS 1,992,822 Granfield Feb. 26, 1935 2,118,137 Bartlett May 24, 1938 2,265,700 Outt Dec. 9, 1941 2,548,624 Sclater Apr. 10, 1951 2,578,395 Brooks Dec. 11, 1951 2,582,351 Olson Jan. 15, 1952 2,771,587 Henderson Nov. 20, 1956 2,947,909 Berger Aug. 2, 1960 FOREIGN PATENTS 570,730 Great Britain July 19, 1945 

1. A REACTIVE TRANSFORMER COMPRISING A MAGNETIC CIRCUIT LOOP FORMED OF A PLURALITY OF LAMINATIONS OF MAGNETIC MATERIAL BENT FLATWISE TO FORM A CLOSED PATH ABOUT A CENTRAL WINDOW, A FLAT STACK OF LAMINATIONS PLACED AGAINST THE SIDE OF SAID CIRCUIT LOOP DEFINED BY THE EDGES OF THE LAMINATIONS OF SAID CIRCUIT LOOP ACROSS SAID CENTRAL WINDOW IN EDGE-TO-EDGE RELATIONSHIP WITH THE LAMINATIONS OF SAID LOOP, SAID LOOP BEING PROVIDED WITH A BUTT JOINT ON ONE SIDE OF SAID STACK OF LAMINATIONS, A PRIMARY ELECTRICAL WINDING ON SAID ONE SIDE OF SAID STACK OF LAMINATIONS AND OVERLYING SAID BUTT JOINT, AND A SECONDARY ELECTRICAL WINDING ON SAID LOOP ON SAID OTHER SIDE OF SAID STACK OF LAMINATIONS. 